1895.] HTDRACHNID rOUTNTD IN COKNWALI,. 199 



The Nervous System (Plate VIII. fig. 20 ; Plate IX. figs. 23, 27). 



I do not know that upon this part of the anatomy I have many 

 observations to describe relative to the present species which differ 

 in very important matters from what has been before observed by 

 other acarologists in various species ; but still I think that there 

 are some new points of considerable interest to be detailed ; and 

 moreover, as former anatomists have not been altogether agreed as 

 to the distribution of the nerves, fresh investigations may be useful, 

 although made upon different species, or even families. 



The great central nervous mass in all Acai'ina which have been 

 investigated is the so-called brain (br.) ; which is penetrated by 

 the oesophagus, that organ passing right through it, generally in a 

 more or less oblique direction, and being accompanied by tracheae in 

 the present species. Although the whole of this brain is one mass, 

 yet its formation from a supra-cesophageal and a sub-oesophageal 

 ganglion is usually fairly apparent ; the latter frequently extend- 

 ing considerably further backward than the former. In the present 

 species the distinction between the upper and lower ganglia 

 is practically lost ; the whole forms one almost, but not quite, 

 globular mass (figs. 20, 23, hr.) which, in the male, has a diameter 

 of about "13 mm. in a dorso-ventral, and of about •! mm. in an 

 antero-posterior direction ; it lies considerably nearer to the ventral 

 than the dorsal surface, indeed its lower edge nearly reaches 

 the ventral cuticle. This brain is situated about as far back as the 

 second pair of legs ; it lies below the salivary glands, and in front 

 of the genital aperture, and is invested by a most distinct neuri- 

 lemma, which is separated from the nervous substance by 

 endosmosis if the organ be soaked in water. The oesophagus 

 (figs. 20, 23, ce.) penetrates the brain in a slightly oblique direction, 

 running backward and a little upward. 



Prom just above the oesophagus there starts from the lower 

 part of the supra-oesophageal portion of the mass a fine, central, 

 azygous nerve (nph.), which runs almost parallel to, but a little 

 above, the oesophagus for the whole length of that organ ; it 

 then splits up into a large number of separate twigs, one of which 

 runs to each muscle of the sucking-pharynx. About this nerve I 

 do not feel any doubt whatever ; I have it in several preparations, 

 and in one fortunate sagittal section of the creature I have the 

 whole length of the nerve from the point where it issues from the 

 brain to its ultimate distribution to the pharyngeal muscles. A 

 precisely similar nerve has been figured by Henkin (op. cit. fig. 7) 

 as existing in Tr-ombidium fulic/inosum. Winkler^ has drawn a 

 similarly placed nerve in Gamasus, but I imagine that he considers 

 that it goes to the lingula, as he calls it the " Zungennerv"; he how- 

 ever identifies it with Henkin's. 



Schaub does not mention any such median nerve as going to the 



^ " Anatomie der Gamasiden," Arbeit, d. zool. Ins. Wien, Tii. p. 336, taf. iii. 

 fig. 8 (1888). 



